Physisorption and Chemisorption

Physisorption and Chemisorption definition

In adsorbed state the adsorbate is held on the surface of adsorbent by attractive forces (bond). Depending on the nature of attractive forces, adsorption can be of two types - physical adsorption (Physisorption) and chemical adsorption (Chemisorption). In chemisorption there is a strong chemical bond.

During adsorption, a new bond is formed between adsorbent and adsorbate. Therefore adsorptions are generally exothermic (▵H = -ve). But entropy and free energy decreases during adsorption.

Enthalpy change during adsorption process are called enthalpy of adsorption. It is defined as the heat evolved at constant pressure, when one mole of an adsorbate is adsorbed on the surface of adsorbent. For physical adsorption and chemical adsorption, its values ranges in the order of -20KJ/mol and 200kjmol-1. Difference between physisorption and chemisorption are given below.

Comparison Between Physisorption and Chemisorption

1. In physisorption adsorbate is held on the surface of  adsorbent by van der Waals force.

In chemisorption molecules of adsorbate and adsorbent are held by chemical bonds.

2. In Physisorption enthalpy of adsorption is comparatively low. ie, in the order of 20 kjmol-1.

 In Chemisorption enthalpy of adsorption is high. ie, in the order of 200 kjmol-1.

3.  Physisorption is reversible.

Chemisorption is irreversible.

4. Physisorption is not specific. ie, all gases are adsorbed by an adsorbent.

Chemisorption is highly specific.

5. In physisorption multi molecular layer of adsorbate occurs at the surface.

Chemisorption forms only unimolecular layer.

6. Physisorption usually takes place at low temperature and the extent of adsorption decreases with increase of temperature.

Chemisorption takes place at relatively higher temperature

7. In physisorption easily liquefiable gases are more adsorbed. For example NH3, HCl etc are more adsorbed than permanent gases like He, O2, N2 etc.

 In Chemisorption there is no such correlation.

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